The ultimate goal of the proposed research is to investigate by quantitative in vitro methods the potential mutagenic effects of regimens used in treatment of pediatric cancer patients. The immediate goal, which is the object of the present proposal, is to determine the frequency of emergence of phenotypically variant peripheral blood lymphocytes before, during, and after combination chemotherapy. Specifically, over a 3-year period, sequential samples of lymphocytes will be obtained from healthy children and from patients with Hodgkin's disease, retinoblastoma, rhabdomyosarcoma, Ewing's sarcoma and Wilms' tumor. The number of 6-thioguanine (TG)-resistant lymphocytes will be counted to determine whether cancer chemotherapy increases the frequency of cells which are resistant, i.e., phenotypic variants. To demonstrate whether or not the TG-resistant variants are in fact mutants, TG-resistant lymphoblastoid lines will be established using a strain of Epstein-Barr virus (B-95-8) that is highly efficient in transforming lymphocytes. All assays in the proposed research will be carried out in vitro, but the changes to be studied are those produced in the patient by the actual chemotherapeutic treatment. The novelty of this proposal, therefore, consists in an attempt to evaluate the mutagenicity of in vivo cancer treatment using rigorously standardized in vitro methods. Potentially the methodology can be applied to the study of chemotherapy-induced mutation as a basis for resistance in tumors and can be expanded to the study of susceptibility to mutation in patients with disorders known to predispose to the development of cancers.